Photoconductive material and process for its preparation



United States Patent 3,345,161 PHOTOCONDUCTIVE MATERIAL AND PROCESS FOR ITS PREPARATION Joseph Marnmino, Rochester, and Willard C. Ham and Margaret C. Pease, Bingharnton, N.Y., assignors to General Aniline & Film Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 13, 1963, Ser. No. 264,759 7 Claims. (Cl. 961.7)

The present invention relates to photoconductive material and a process for its preparation. It has particular reference to a process for the optical sensitization of photoconductors of variou kinds, and particularly the more commonly used materials such as zinc oxide for use in electrophotography. Electrophotography is a wellknown process for converting a light image into .an electrostatic charge pattern on a sheet or support having an electrically insulating layer of material which is also photoconductive. In this process, a latent image is first produced, which can be developed into a visible image, resulting in reproduction of the original pattern.

A typical electrostatic reproduction process includes use of a recording member comprising a backing or support and bearing a layer or coating which normally is an insulator but which, on exposure to actinic radiation, becomes at least somewhat conductive. An electrostatic charge is imparted to the backing and/or the surface of the photoconductive layer, establishing a potential between them. A light image is then focused on the charged surface. The charge is dissipated in the exposed areas or partially dissipated depending on the amount of exposure, while the unexposed areas remain charged. A pattern of electrostatic charges results which correspond to the light image. The latent image may be rendered visible by applying thereto particles of charged electroscopic powder. The charged particles are held by electrostatic forces to charged areas of the sheet.

Electrostatic photography and electrostatic printing, with both of which this invention is concerned, commonly use as a photosensitive element, a sheet of film, paper, or the like hearing a thin photoconductive insulating layer. This layer includes not only photoconductive materials such as zinc oxide, but also includes a resin, such as an organic polymer, resin or natural rosin or the like for dark insulating purposes. These materials are applied by appropriate methods over a suitable base sheet. The base sheet material is sometimes selected for its own electroconductive properties, in which case tlTe base normally will be connected to ground potential during the processing. Highly conductive bases of this type have been prepared from sheet aluminum, carbon filled paper and the like. Alternatively, it is possible to use relatively nonconductive base materials, such as untreated paper or paper suitably filled or otherwise treated. In the latter case, in order to apply an electrostatic pattern, a charge must be imparted to the base material, which charge has a polarity different from or opposite to that to be applied on the photoconductive surface. In other words, a sheet is coated with a layer of the photoconductive material, for example zinc oxide; the zinc oxide being supported in, mixed with, or covered by a resin or the like. This layer normally provides an effective insulator so that a charge of one polarity may be imparted to the base sheet and a different charge which may be neutral or of opposite polarity is applied to the top of the resin film or photoconductive layer.

Therefore, in the practice of this form of electrophotography, it is customary to deposit an electrostatic charge, usually uniform over the whole area on the face of the photoconductive insulating layer by some suitable means 3,345,161 Patented Oct. 3, 1967 in the absence of actinic illumination. Meanwhile, the backing layer is either connected to ground or has applied thereto an approximately equivalent charge of opposite polarity. The photoconductor is then illuminated by the light and shadow pattern which represents the image to be reproduced. The necessary illumination for creating the latent image may be obtained in various ways, as by exposure to the output of an oscilloscope or equivalent, or exposure to a projector or camera or the like. Wherever the image is illuminated by actinic radiation, the hitherto insulating photoconductive material becomes electrically conductive or at least semi-conductive. Hence, any previously deposited electrostatic surface charge is dissipated partly or entirely by this conductivity, generally according to the greater or lesser actinic exposure which each element of area has received. In this manner, the electrostatic latent image is formed with the greatest charge density remaining in the unexposed areas, and the least charged density remaining in those portions most highly illuminated.

As previously mentioned, the resulting latent image must be rendered visible by development which can be accomplished in several different ways. Most commonly, electrostatically charged powdered particles of a pigment or so-called toners are applied to the latent image. This application may be in the form of airborne clouds of pow der or the particles may be suspended in insulating liquids of relatively low dielectric properties. They also may be applied as dry particles adhering to oppositely charged carrier particles of substantially larger size. Carrier particles such as iron powder or very fine glass beads may be used for this purpose.

Depending on the relative electric charge or polarities of the electrostatic latent image and the development pigment or powder particles, the latter adhere differentially to charged, partially charged and discharged areas, thereby rendering them differentially visible. The image is then in a typical xerographic operation, transferred by contact to a receiving sheet and is afiixed thereto. Alternatively, the pigment may be directly aflixed to the photoconductive layer by drying to form a residue, by thermal fusion, or by the action of solvent vapor. The general details of this process are well known to those skilled in the art and need not be further elaborated here.

Various photoconductive materials are known which can be used in this general process. Among them are such materials as the colored oxides, sulfides, selenides, tellurides and sometimes the iodides of various metals including cadmium, mercury, antimony, bismuth, thallium, indium, molybdenum, aluminum, lead, and zinc. Certain other colored salts, such as arsenic trisulfide, cadmium arsenide and lead chromate also have photoconductive properties. Among all these, however, the oxides of zinc are generally preferred for reasons of economy, because they respond well to electrostatic charges and they have a good wide range of light to dark conductivities, because they form a very useful white surface of good texture, and because they are widely available in suitable form. Certain dyestuffs can be used with zinc oxide to increase its sensitivity to visible radiation. Those dyes which are capable of absorbing visible radiant energy and of transferring the absorbed energy to the zinc oxide itself, are most useful. A large number of dyestuffs have been suggested for this purpose but most of them have serious deficiencies. In the first place, the color of the Zinc oxide is white and it is not usually desirable to have this whiteness covered by deep colors. The handling of zinc oxide coatings containing the dyestuffs themselves in their highly colored state, may cause difliculties. For this and other reasons, various investigators have suggested ways and means to use dyestuifs with various treatments to make zinc oxide a more suitable photoconductive material for the type of process in question.

While zinc oxide is usually preferred as the pigment of the photoconductive insulator to be used in the production of electrophotographic images, as indicated above, it has certain deficiencies including a relatively insensitive to visible radiation. The peak sensitivity of zinc oxide lies between 2,350 to about 3,900 Angstroms. Various investigators have recognized that it is possible to sensitize zinc oxide to visible radiation by adsorbing suitable dyes upon its surface. This sort of sensitization renders the zinc oxide responsive to colored light and greatly increases its effective sensitivity to visible radiation. The mechanism of optical sensitization is not yet clearly understood but it is believed that the sensitizer must be intimately associated and perhaps adsorbed directly on the crystal surface.

It has been recognized by the prior art that it might be desirable to treat the photoconductive zinc oxide by adsorbing the dyes or other optical sensitizing materials firmly onto the powder or crystals prior to incorporating the zinc oxide into the resinous layer or before applying a coating which sup-ports or covers the Zinc oxide layer. In practice, this may be done by suspending the treated and dyed zinc oxide in a solution of the insulating resin which is dissolved in an aromatic or chlorinated aromatic solvent. Since dyes may be chosen which are insoluble in such resins, they may be effectively adsorbed to the zinc oxide. However, the use of such solvents present serious problems. Most of the solvents are toxic, and some are flammable. Consequently, the safe removal of these solvents presents a serious problem. Moreover, the solvents are expensive and their recovery usually is not possible. Even their safe disposal is an expensive or troublesome operation. It will, therefore, be realized that it would be highly desirable to utilize aqueous dispersions of the resin instead.

It has already been suggested in the prior art, for instance, in French Patent 1,249,167 that certain resins, especially those derived from maleic anhydride and related materials, might be dissolved or emulsified or otherwise dispersed in aqueous solution to obtain the desired result of a resin which suitably supports the zinc oxide and which gives no problems of toxicity and fire hazards in its handling by inexperienced personnel. However, as suggested above, materials of this type, particularly if they use a pre-dyed zinc oxide, are relatively highly colored, contaminating to their surroundings, and are relatively poorly sensitized. The dyes tend to deposit largely in the resin matrix, rather than adhere to the zinc oxide itself, and are, therefore, not as effective as desired.

It is accordingly an object of the present invention to provide a novel process of optical sensitization which is particularly adapted to the sensitization of zinc oxide without being limited to this particular photoconductive material. Other objects will be apparent from the follow- In general terms, our novel process consists of heating the dry zinc oxide pigment with a suitable dye at a sufficiently elevated temperature and for a sufficient period of time to cause a surface reaction between the dye and the zinc oxide. The temperature is preferably so high that the chemistry of the dye is somewhat altered. Hence, the reaction takes place between zinc oxide and an oxidation product or a residue of the dye rather than the dye itself. The more common classes of sensitizing dyes can be used in this way. On heating with the zinc oxide, the dyes become very light in color, frequently quite colorless regardless of the initial color of the dye. Nevertheless, effective combinations of materials which are obtained consist, not of the original dyes, but of oxidation products combining with the zinc oxide. To describe the process in greater detail, a specific example is given.

Five samples were prepared each containing 100 mg. of Rhodamine B Extra and 100 grams of zinc oxide, USP XII grade. This mixture was thoroughly mixed to insure an even distribution of the dye over the zinc oxide. One portion was set aside as a control sample and the remaining samples were heated for various times and at varying temperatures. In each case, after heating, the sample was checked for its dye-solubility in water. In addition, a 100-gram control sample of zinc oxide without dyestuff was heated to 640 F. for 6 minutes. This was the same time and temperature as required for the pigment dye samples which showed dye insolubilization.

Electrophotographic coatings were then made, using the treated zinc oxide, i.e., zinc oxide treated with dye at high temperature, and a resin emulsion of a water dispersible epoxy ester-vinyl acetate crotonic acid copolymer manufactured by Washburn & Co. under the trade name Xeroplex #1. The mixture of zinc oxide, treated as above using a pigment to binder ratio of 3.5 to l, was ball-milled for 27 hours to insure complete mixing. Thereafter, a clay-coated bond paper, identified as .Crocker-Burbank T977, was coated with the zinc oxide and resin emulsion using a #18 wire-bound bar. Coatings were dried and the various samples were then tested in the manner next described.

A negative electrostatic charge was deposited upon the face of the photoconductive insulating layer of paper by means of a corona discharge device in the absence of actinic illumination. The photoconductor was then illuminated in contact with a film positive, using a General Electric CDJ 100-watt tungsten light bulb mounted 30 inches above the image and operated at a color temperature of about 2950 F. The latent image thus formed was rendered visible by a conventional positive blue liquid toner of the type described in US. Patent 2,907,674. The sensitivity of the coating was determined by examining the resulting print for background toner pick-up, sharpness of image and image fill-in. The exposure time was adjusted by experiment to get a sharp image in each case. The results are summarized below:

TABLE I Heating Temp. Expo- Sample Time, F.) Iteacted Dye Pigment Color sure Min. End of Solubility in 11 0 Time,

Heating Sec.

A. Undyed ZnO White 4 B. Undyed ZnO 6 640 .do 4 C. 100 mg. dye/100 g. ZnO None Very soluble Magenta 3 D. 100 mg. dye/100 g. ZnO 1 310 Partially s0luble Pink 1. 5 E. 100 mg. dye/100 g. ZnO 4 550 Slightly soluble.-. Pinkish white 1. 5 F. 100 mg. dye/100 g. ZnO 5 600 Not soluble Slight white-pink hue... 1. 5 G. 100 mg. dye/100 g. ZnO 6 640 ..do White 2.3

ing descriptions. This process causes the sensitizer apparently to form a less water-soluble material in conjunction with the zinc oxide. At the same time, the resulting product is not highly colored.

As shown in the above table, the coatings of undyed zinc oxide A and B showed normal behavior for an unsensitized electrophotographic coating. Coating C was slightly sensitized in comparison to A and B, but was highly colored and was considered objectionable for this reason. Coatings D, E and F showed an increase in sensitivity along with increased reductions of the original dye color. They also showed a reduction in dye solubility in the resin binder. It appears that optimum sensitization along with the desired White color is achieved best between the temperatures of steps F and G, which correspond to about the upper practical limit. Within this range, the required exposure for a good image was at a minimum, indicating maximum sensitization. At the same time, there was little or no background coloration.

The quantity of dye which is used to achieve sensitization may be as high as 0.5% based on the weight of the zinc oxide. It is preferred, however, to use lesser amounts and in many instances, it is advantageous to use the minimum amount which is capable of producing the desired results. A range of 1 to parts per 1000 is considered to be the practical limit, although 1 to 2 parts per 1000 is most commonly preferred.

Various classes of dyes can be used with our invention. All of these dyes form essentially water insoluble materials With zinc oxide when heated with the zinc oxide to the temperatures and time intervals described.

TABLE II Preferred Preferred Temper- Thermoature fusion Range, F. Time, min.

I. Acid Dyes:

Patent Blue 600-670 5 Rose Bengal 550-600 5-7 Eosin 450-550 6-6 Erythrosin -7 Fluoreseein 450-550 6. II. Basie Dyes: -8

Malachite Green 400-500 8 Brilliant Green 450-500 3-5 Auramine 580-650 4-5 Crystal Violet 480-550 3-6 Rhodamine 550-600 4-5 III. Anthraquinone Dye. Ce ton Fast -5 Pink 650-700 4-6 IV. sulfonphthalein Dyes:

Brornophenol Blue 480-530 6 Bromocresol Green 580-650 4 It will be noted from the foregoing description that the present invention makes it possible to improve the sensitivity'of zinc oxide to visible light without sacrificing the desirable white Zinc OXide background. Besides, the dyestufi is not leached away. Moreover, the use of dangerous or potentially dangerous solvents is avoided. All of these advantages make the process more acceptable for use in ofiices where technically untrained personnel must operate the reproduction equipment.

It will be understood from the foregoing description that other dyestuffs may be used and that metallic oxides or sulfides other than the zinc oxide shown in the examples may be treated in the same general manner. These and other obvious variations will be readily apparent to those skilled in the art.

What is claimed is:

1. A sensitized photoconductive element comprising a base sheet coated with a photoconductive material preheated in dry form with a dyestufi' selected from the group consisting of acid dyes, basic dyes, anthraquinone 6 dyes and sulfonphthalein dyes, to a temperature frorr 400 to 650 F. sufficiently high to oxidize the dyestufl and react with said material whereby the coloring effect and aqueous resin emulsion solubility of said dyestutf is reduced.

2. A process for the optical sensitization of white photoconductive materials normally insensitive to visible light radiation but sensitive to ultraviolet light, which process comprises intimately mixing with said material, an organic dyestulf selected from the group consisting of acid dyes, basic dyes, anthraquinone dyes and sulfonphthalein dyes; heating the intimate mixture in dry form to a temperature from 400 to 650 F. suificiently high to permit an oxidation product of said dyestufi' to be formed and a reaction to occur between said material and said oxidized product thereby rendering the dyestuff less soluble in solvents; and dispersing the reacted material in an aqueous resin emulsion.

3. A process according to claim 2, wherein the photoconductive material is zinc oxide.

4. A process according to claim 2, wherein the water soluble dyestuff is Rhodamine B.

5. A process according to claim 2, wherein the quantity of dyestulf is from 1 to 10 parts per 1000 parts of photoconductive material.

6. The process of sensitizing zinc oxide to visible light radiation which comprises adding to zinc oxide a dye sensitizer selected from the group consisting of acid dyes, basic dyes, anthraquinone dyes and sulfonphthalein dyes, intimately mixing said zinc oxide and sensitizer and heating the mixture in dry form to a temperature ranging from 400 to 650 F. for at least 1 minute whereby said dye sensitizer is oxidized and reacted with said zinc oxide, the dye solubility in an aqueous resin emulsion, is reduced and the dye color is substantially destroyed to leave an essentially white product.

7. The process of preparing a material useful in electrophotography which comprises coating a base with an aqueous resinous coating containing in suspension the reaction product of zinc oxide and an oxidized dyestuif selected from the group consisting of acid dyes, basic dyes, anthraquinone dyes and sulfonphthalein dyes, obtained by heating the zinc oxide and the dyestuff in dry form to an elevated temperature from 400 to 650 F. for a suflicient period of time to form an essentially water insoluble complex, drying the coating thus applied to deposit a resinous layer of the zinc oxide-dye complex on the base.

References Cited UNITED STATES PATENTS 3,051,569 8/1962 Sugarman et al 96-1 3,052,540 9/1962 Greig 96-1 3,121,007 2/1964 Middleton et a1 96-1 3,128,179 4/ 1964 Kendall et a1. 96-1 3,203,795 8/1965 Schaum et a1 96-1.7 3,238,149 3/1966 Spurr 96-1.7 X

FOREIGN PATENTS 919,684 2/1963 Great Britain.

NORMAN G. TORCHIN, Primary Examiner. D. D. PRICE, C. E. VAN HORN, Assistant Examiners. 

1. A SENSITIZED PHOTOCONDUCTIVE ELEMENT COMPRISING A BASE SHEET COATED WITH A PHOTOCONDUCTIVE MATERIAL PREHEATED IN DRY FORM WITH A DYESTUFF SELECTED FROM THE GROUP CONSISTING OF ACID DYES, BASIC DYES, ATHRAQUINONE DYES AND SULFONPHTHALEIN DYES, TO A TEMPERATURE FROM 400 TO 650*F. SUFFICIENTLY HIGH TO OXIDIZE THE DYESTUFF AND REACT WITH SAID MATERIAL WHEREBY THE COLORING EFFECT AND AQUEOUS RESIN EMULSION SOLUBLITY OF SAID DYESTUFF IS REDUCED. 